[18F] fluoride cryptate complexes for radiolabeling fluorinations

ABSTRACT

The present invention claims UV detectable (λ&gt;210 nm) potassium [18F]fluoride diaryl- and aryl-fused [2.2.2]cryptate complexes suitable for performing radio-labeling reactions to generate [18F] fluorinated species.

CROSS-REFERENCE TO RELATED APPLICATION

The disclosure is a continuation of U.S. patent application Ser. No.15/731,330, filed May 30, 2017, which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to UV detectable (λ>210 nm) potassium[¹⁸F]fluoride diaryl- and aryl-fused [2.2.2]cryptate complexes suitablefor performing radio-labeling reactions to generate [¹⁸F] fluorinatedspecies for use as imaging agents.

A cryptand is a phase-transfer agent used to complex [¹⁸F] fluorideanion to form [¹⁸F] fluoride cryptate complexes and that a [¹⁸F]fluorinated species defined herein comprises chemical or biological[¹⁸F] fluorinated compounds.

BACKGROUND OF THE INVENTION

Positron Emission Tomography (PET) relies upon the use of positronemitting radiolabeled tracer molecules and computed tomography toexamine metabolic processes or to detect targets within the living bodyof a patient or experimental animal. Once injected, the tracer ismonitored with a positron camera or a tomograph detector array. Thistechnology can be more sensitive than scanning techniques such asmagnetic resonance imaging (MRI), ultrasound imaging, or X-ray imaging.Some of the major clinical applications for PET are oncology, neurology,and cardiology. Positron emitting compounds may be employed as markersand imaging agents because their presence and location are indicated bythe annihilation of a nearby electron and the consequent emission of twooppositely oriented gamma rays. Gamma ray detectors can be used todetect the event and precisely determine its location. Tracer moleculesused in PET imaging are generally prepared by replacement of a group oratom in an unlabeled tracer with a radioisotope containing group or atomor by joining the tracer to a radioisotope containing atom (e.g. bychelation). Some common positron-emitting radioisotopes commonly usedare: fluorine-18 (¹⁸F); carbon-11 (¹¹C); nitrogen-13 (¹³N); andoxygen-15 (¹⁵O). In addition, ⁶⁴Cu has been appended to tracer moleculesusing copper chelation chemistry (Chen et al. Bioconjug. Chem. (2004)15: 41-49).

¹⁸F is a particularly desirable radioisotope for PET imaging since ithas a longer half-life than ¹¹C, ¹³N and ¹⁵O, readily forms covalentbonds, and has a short range beta+ emission that provides for highresolution in PET imaging. Natural, stable fluorine is ¹⁹F. ¹⁸F has oneless neutron for that number of protons, which is why it decays bypositron emission.

¹⁸F is a fluorine radioisotope which is an important source ofpositrons. It has a mass of 18.0009380 u and its half-life is 109.771minutes. It decays by positron emission 97% of the time and electroncapture 3% of the time. Both modes of decay yield stable oxygen-18(¹⁸O). ¹⁸F is an important isotope in the radiopharmaceutical industry.For example, it is synthesized into fluorodeoxyglucose (FDG) for use inpositron emission tomography (PET scans). It is substituted for hydroxyland used as a tracer in the scan. Its significance is due to both itsshort half-life and the emission of positrons when decaying.

In the radiopharmaceutical industry, the radioactive ¹⁸F must be madefirst as the fluoride anion (¹⁸F⁻) in the cyclotron. This may beaccomplished by bombardment of neo-20 with deuterons, but usually isdone by proton bombardment of ¹⁸O-enriched water, with high energyprotons (typically ˜18 MeV protons). This produces “carrier-free”dissolved ¹⁸F-fluoride (¹⁸F⁻) ions in the water. Fluorine-18 is oftensubstituted for a hydroxyl group in a radiotracer parent molecule. PETtracers often are or include a molecule of biological interest (a“biomolecule”). Biomolecules developed for use in PET have beennumerous. They can be small molecules as ubiquitous as water, ammoniaand glucose or more complex molecules intended for specific targeting inthe patient, including labeled amino acids, nucleosides and receptorligands. Specific examples include, but not limited to, ¹⁸F labeledfluorodeoxyglucose, methionine, deoxythymidine, L-DOPA, raclopride andFlumazenil. (Fowler J. S. and Wolf A. P. (1982), and The synthesis ofcarbon-11, fluorine-18 and nitrogen-13 labeled radiotracers forbiomedical applications. Nucl. Sci. Ser. Natl Acad. Sci. Nal Res.Council Monogr. 1982).

The 109.8 minute half-life of ¹⁸F makes rapid and automated chemistrynecessary after this point. ¹⁸F-fluoride anion (¹⁸F⁻) is often convertedto a form suitable as an agent in aliphatic nucleophilic displacementsor aromatic substitution reactions. ¹⁸F may be combined with a metal ioncomplexing agent such as cryptand or a tetrabutyl ammonium salt, atriflate, or a positively charged counter ion (including H⁺, K⁺, Na⁺,etc).

Fluorination agents may be used in an appropriate solvent or cosolvent,including without limitation water, methanol, ethanol, THF,dimethylformamide (DMF), formamide, dimethylacetamide (DMSO), DMA,dioxane, acetonitrile, and pyridine.

In nucleophilic radiofluorination, the first major step is drying theaqueous [¹⁸F] fluoride which is commonly performed in the presence of aphase-transfer cataylst under azeotropic evaporation conditions (Coenenet al., J. Labelled Compd. Radiopharm., 1986, vol. 23, pgs. 455-467).The [¹⁸F] fluoride that is solubilized or dissolved in the target wateris often adsorbed on an anion exchange resin and eluted, for example,with a potassium carbonate solution (Schlyer et al., Appl. Radiat.Isot., 1990, vol. 40, pgs. 1-6). One cryptatnd that is availablecommercially is 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo [8,8,8]hexacosan, with the tradename Kryptofix 222. Cryptand is a cage-likeagent that has three ether ribs joining the nitrogens at each end.Alkali metals can be held very strongly inside the cage. The treatmentwith ¹⁸F⁻ is suitably effected in the presence of a suitable organicsolvent such as acetonitrile, dimethylformamide, dimethylsulphoxide,tetrahydrofuran, dioxan, 1,2 dimethoxyethane, sulpholane,N-methylpyrolidinineone.

In nucleophilic fluorination reactions, anhydrous conditions arerequired to avoid the competing reaction with hydroxide. [Aigbirhio etal 1995 J. Fluor. Chem. 70 pp 279-87]. The removal of water from thefluoride ion is referred to as making “naked” fluoride ion. This isregarded in the prior art relating to nucleophilic fluoridation as astep necessary to increase the reactivity of fluoride as well as toavoid hydroxylated by-products resulting from the presence of water[Moughamir et al 1998 Tett. Letts. 39 pp 7305-6; and Handbook ofRadiopharmaceuticals 2003 Welch & Redvanly eds. ch. 6 pp 195-227). Theremoval of water from the [¹⁸F] Fluoride is referred to as making“naked” [¹⁸F] Fluoride. This is regarded in the prior art relating tonucleophilic fluoridation as a step necessary to increase the reactivityof fluoride as well as to avoid hydroxylated by-products resulting fromthe presence of water (Moughamir et al 1998 Tett Letts; 39: 7305-6).

The use of the cryptand to sequester the potassium ions avoidsion-pairing between free potassium and fluoride ions, making thefluoride anion more reactive. For example, [(2.2.2-cryptand) K⁺] ¹⁸F⁻ isreacted with a protected mannose triflate; the fluoride anion displacesthe triflate leaving group in an S_(N) ² reaction, giving the protectedfluorinated deoxyglucose. Base hydrolysis removes the acetyl protectinggroups, giving the desired product ¹⁸FDG after removing the cryptand viaion-exchange (Fowler J S, Ido T (2002). “Initial and subsequent approachfor the synthesis of ¹⁸FDG”. Semin Nucl Med 32 (1): 6-12; and Yu, S(2006). “Review of ¹⁸F-FDG synthesis and quality control”. BiomedicalImaging and Intervention Journal 2).

To improve the reactivity of fluoride ion for fluoridation reactions acationic counterion is added prior to the removal of water. Thecounterion should possess sufficient solubility within the anhydrousreaction solvent to maintain the solubility of the fluoride ion.Therefore, counterions that have been used include large but soft metalions such as rubidium or caesium, potassium complexed with a cryptandsuch as Kryptofix™, or tetraalkylammonium salts. A preferred counterionfor fluoridation reactions is potassium complexed with a cryptand suchas Kryptofix™, because of its good solubility in anhydrous solvents andenhanced fluoride reactivity.

Fluorodeoxyglucose (¹⁸F) or fludeoxyglucose (¹⁸F), commonly abbreviated¹⁸F-FDG or FDG, is a radiopharmaceutical used in the medical imagingmodality positron emission tomography (PET). Chemically, it is2-deoxy-2-(¹⁸F) fluoro-D-glucose, a glucose analog, with thepositron-emitting radioactive isotope fluorine-18 substituted for thenormal hydroxyl group at the 2′ position in the glucose molecule.Synthesis of the FDG itself is not considered to be part of thisinvention and only a basic description of a process is included here.

Production of ¹⁸-labeled FDG is, by now, well known. Information can befound in: 1) Fowler et al., “2-Deoxy-2-[¹⁸F]Fluoro-D-Glucose forMetabolic Studies: Current Status,” Appl. Radiat. Isotopes, vol. 37, no.8, pp. 663-668 (1986); 2) Hamacher et al., “Efficient StereospecificSynthesis of No-Carrier-Added 2-[¹⁸F]-Fluoro-2-Deoxy-D-Glucose UsingAminopolyether Supported Nucleophilic Substitution,” J. Nucl. Med., vol.27, pp. 235-238 1986; 3) Coenen et al., “Recommendation for PracticalProduction of [2-¹⁸F]Fluoro-2-Deoxy-D-Glucose,” Appl. Radial Isotopes,vol. 38, no. 8, pp. 605-610 (1987) (a good review); 4) Knust et al.,“Synthesis of ¹⁸F-2-deoxy-2-fluoro-D-glucose and¹⁸F-3-deoxy-3-fluoro-D-glucose with no-carrier-added ¹⁸F-fluoride,” J.Radioanal. Nucl. Chem., vol. 132, no. 1, pp. 85-91 (1989); and 5)Hamacher et al. “Computer-aided Synthesis (CAS) of No-carrier-added2-[¹⁸F]Fluoro-2-Deoxy-D-Glucose: An Efficient Automated System for theAminopolyether-supported Nucleophilic Fluorination,” Appl. Radiat.Isotopes, vol. 41, no. 1, pp. 49-55 (1990). See also U.S. Pat. No.6,567,492 to Kislelev al. (20 May 2003).

Several automatic processing systems capable of production ofradiopharmaceuticals, such as ¹⁸F-labeled FDG, have also been describedin: 1) U.S. Pat. No. 5,808,020 to Ferried et al. (15 Sep. 1998); 2) U.S.Pat. No. 6,599,484 to Zigler et al. (29 Jul. 2003); PCT pub.WO2004093652 by Buchanan et al. (2004 Nov. 4); and 3) German patentDE10320552 to Maeding et al. “Apparatus marking pharmaceuticalsubstances with fluorine isotope, preparatory to positron-emissiontomography, locates anion exchanger within measurement chamber” (2004Nov. 25). Clinical Use of ¹⁸F-FDG.

¹⁸F-FDG, as a glucose analog, is taken up by high-glucose-using cellssuch as brain, kidney, and cancer cells, where phosphorylation preventsthe glucose from being released again from the cell, once it has beenabsorbed. The 2′ hydroxyl group (—OH) in normal glucose is needed forfurther glycolysis (metabolism of glucose by splitting it), but ¹⁸F-FDGis missing this 2′ hydroxyl. Thus, in common with its sister molecule2-deoxy-D-glucose, FDG cannot be further metabolized in cells. The¹⁸F-FDG-6-phosphate formed when ¹⁸F-FDG enters the cell thus cannot moveout of the cell before radioactive decay. As a result, the distributionof ¹⁸F-FDG is a good reflection of the distribution of glucose uptakeand phosphorylation by cells in the body. After ¹⁸F-FDG decaysradioactively, however, its 2′-fluorine is converted to ¹⁸O⁻, and afterpicking up a proton H⁺ from a hydronium ion in its aqueous environment,the molecule becomes glucose-6-phosphate labeled with harmlessnonradioactive “heavy oxygen” in the hydroxyl at the 2′ position. Thenew presence of a 2′ hydroxyl now allows it to be metabolized normallyin the same way as ordinary glucose, producing non-radioactiveend-products.

After ¹⁸F-FDG is injected into a patient, a PET scanner can form imagesof the distribution of FDG around the body. The images can be assessedby a nuclear medicine physician or radiologist to provide diagnoses ofvarious medical conditions.

In PET imaging, ¹⁸F-FDG can be used for the assessment of glucosemetabolism in the heart, lungs, and the brain. It is also used forimaging tumors in oncology, where a static ¹⁸F-FDG PET scan is performedand the tumor ¹⁸F-FDG uptake is analyzed in terms of Standardized UptakeValue (SUV). ¹⁸F-FDG is taken up by cells, phosphorylated by hexokinase(whose mitochondrial form is greatly elevated in rapidly growingmalignant tumours), and retained by tissues with high metabolicactivity, such as most types of malignant tumours. As a result FDG-PETcan be used for diagnosis, staging, and monitoring treatment of cancers,particularly in Hodgkin's disease, colorectal cancer, breast cancer,melanoma, lung cancer, and Alzheimer's disease.

Cryptands

Cryptands and other macrocyclic compounds such as crown ethers,spherands, cryptahemispherands, cavitands, calixarenes, resorcinorenes,cydodextrines, porphyrines and others are well known. (ComprehensiveSupramolecular Chemistry Vol. 1-10, Jean-Marie Lehn-Chairman of theEditorial Board, 1996 Elsevier Science Ltd.) Many of them are capable offorming stable complexes with ionic organic and inorganic molecules.Those properties make macrocyclic compounds candidates for variousfields, for instance, catalysis, separations, sensors development andothers. Cryptands (bicyclic macrocycles) have extremely high affinity tometal ions. The cryptand metal ion complexes are more stable than thoseformed by monocyclic ligands such as crown ethers (Izatt, R. M., et al.,Chemical Reviews 91:1721-2085 (1991)). This high affinity of thecryptands to alkaline and alkaline earth metal ions in water makes themsuperior complexing agents for the processes where strong, fast andreversible metal ion binding is required. Examples of these processesinclude separation, preconcentration and detection of metal ions,analysis of radioactive isotopes, ion-exchange chromatography,phase-transfer catalysis, activation of anionic species and others.

Many strategies for the synthesis of macrocyclic compounds have beendeveloped over the years (Comprehensive Supramolecular Chemistry Vol.1-10, Jean-Marie Lehn-Chairman of the Editorial Board, 1996 ElsevierScience Ltd.; Krakowiak, K. E., et al., Israel Journal of Chemistry32:3-13 (1992); Bradshaw, J S., et al., “Aza-Crown Macrocycles,” TheChemistry of Heterocyclic Compounds, Vol. 51, ed. Taylor, E. C., Wiley,New York, 1993; Haoyun, A., et al., Chemical Reviews 92:543-572 (1992)).

The Cryptands may be synthesised as described in US20040267009 A1,Bernard Dietrich, Jean-Marie Lehn, Jean Guilhem and Claudine Pascard,Tetrehedron Letters, 1989, Vol. 30, No. 31, pp 4125-4128, Paul H. Smithet al, J. Org. Chem., 1993, 58, 7939-7941, Jonathan W. Steed et al,2004, Journal of the American Chemical Society, 126, 12395-12402,Bing-guang Zhang et al, Chem. Comm., 2004, 2206-2207.

Cryptands are cavity containing macromolecules which form stablecomplexes with alkali metal ions. For a given cation, the stabilityconstant is largest for the cation which fits best into the cavity ofthe ligand. Thus stability maxima are found for Li[2.1.1]⁺, Na[2.2.1]⁺,and K[2.2.2]⁺ (Cox, B. G. Effects of substituents on the stability andkinetics of alkali metal cryptates in methanol. Inorganica ChimnricaActa, 1981, 49, 153-158).

Substituted [2.2.2] cryptands, such as dibenzo[2.2.2] cryptand (VII)possess a guest binding site (ionophore) having heteroatom Withnonbonding electron pairs such as nitrogen, capable of binding potassium(K⁺) selectively in its cavity. VII as phase transfer reagent (PTR) inthe synthesis of [18F]fluoride cryptate complexes for radiolabelingfluorinations will have improved detectability which will facilitatereliable assessment of PTR in the emerging direction of automated QCtesting platforms. VII has strong UV absorbance at λ>210 nm wavelength.Molar absorptivity values for VII are high across a wide range of pH,4100 M⁻¹ cm⁻¹ at pH 2.4-3.0 (272 nm), and 4400 M⁻¹ cm⁻¹ at pH 6.2-6.6(276 nm).

Molar absorptivitv can be calculated from the equation: A=εcl, where Ais the absorbance at λ_(max) at 272 nm, ε is the molar absorptivity(M⁻¹cm⁻¹), c is the concentration (M), and 1 is path length (1 cm).

Molar absorptivitv of K222BB can be calculated from data given in thegraph of “Absorbance versus wavelength for K-222BB, from 0.18 mM-0.068mM”

C (mM) A ε 0.18 0.76 42222 0.16 0.65 40625 0.14 0.58 41429 0.12 0.541667 0.11 0.46 41818 0.1 0.42 42000 0.08 0.33 41250 0.07 0.3 42857

K-222 has its absorbance maximum ˜200 nm where there is significantissues with solvent interference.

Table 11-9 of LAMBERT (Organic Structural Spectroscopy, 1998, pages287-289) lists very small molar absorptivity 205 M⁻¹cm⁻¹ at 254 nm ofbenzene in water, and 170 M⁻¹ cm⁻¹ at 257 nm of bromobenzene in ethanol.

VII (K-222BB) shows lambda max at 272 m, a wavelength with nointerference from solvents. For example, FIGS. 1 and 2 show absorptionspectra for acetonitrile and methanol (commercial HPLC type and specialgrade), indicating 210 nm of their UV cutoff (see Tips for practicalHPLC analysis—Separation Know-how—Shimadzu LC World Talk Special IssueVolume 2; page 6).

JOHNSON (U.S. Pat. No. 5,264,570, issued 23 Nov. 1993) compared therecovered [¹⁸F]FDG made by the method using Kryptofix K222BB to themethod of the prior art using Kryptofix K222 with respect to residualtraces of the phase-transfer reagent in the final [¹⁸F]FDG product. Theyemployed TLC and HPLC techniques. JOHNSON describes a series of columnswas used to analyze the prepared [¹⁸F]FDG to determine wt % of phasetransfer reagent (PTR) present. The product was passed through a seriesof columns. Using this procedure, JOHNSON found that Kryptofix K222 waspresent at 30-50% by weight of the initial charge. The Kryptofix K222BBwas found to be present at 5-7%. JOHNSON did not describe UV detectionof K222BB at λ>210 nm. JOHNSON compared residual traces of K222BB toresidual traces of K222 which has no UV absorption at λ>210 nm.

Nakao et al. (Simultaneous analysis of FDG, CIDG and Kryptofix 2.2.2 in[¹⁸F]FDG preparation by high-performance liquid chromatography with UVdetection. Nuclear Medicine and Biology 35 (2008) 239-244) showedKryptofix 2.2.2 (K-222) dissolved with 50 mM ammonium phosphate bufferat different pH values absorbs light at ca. 200 nm under a neutral oralkaline condition (FIG. 3A). FIG. 3 showed K-222 displays peakabsorption at ca. 210 nm at pH 9.3. Nakao et al. also described HPLCanalysis of K-222 at 210 nm (see Abstract and page 240, section 2.3 ofNakao's paper).

JONSON (U.S. Pat. No. 5,264,570) described hydrolysis of [18F]fluorideion substituted triflate was achieved by adding 2N HCl. Afterhydrolysis, the solution was passed through a chain of columns, one ofwhich is to neutralize the product mixture.

JOHNSON compared the recovered [¹⁸F]FDG made by using Kryptofix K222BBto the method of the prior art using Kryptofix K222 with respect toresidual traces of the phase-transfer reagent in the final [¹⁸F]FDGproduct. They employed TLC and HPLC techniques. JOHNSON describes aseries of columns was used to analyze the prepared [¹⁸F]FDG to determinewt % of phase transfer reagent (PTR) present. The product was passedthrough a series of columns. Using this procedure, JOHNSON found thatKryptofix K222 was present at 30-50% by weight of the initial charge.The Kryptofix K222BB was found to be present at 5-7%. JOHNSON did notdescribe UV detection of K222BB at λ>210 nm. JOHNSON compared residualtraces of K222BB to residual traces of K222 which has no UV absorptionat λ>210 nm, as shown by Nakao's study above, indicating they comparedK222BB with K-222 at λ<210 nm.

To support the limitation “UV detectable (λ>210 nm),” the absorbanceversus wavelength for K-222BB, from 0.18 mM-0.068 mM at pH=3 is given inFIG. 1. K-222BB shows lambda max at 272 nm, a wavelength with nointerference from solvents.

The advatages of ary-fused[2.2.2]cryptands as phase transfer reagents inthe synthesis of [18F]fluoro-pharmaceuticals are: (1) they can bedetected and tested at wavelengths (λ>210 nm) without solventintereference; (2) they have stronger UV absorbance (molar absorptivityε>1000 M⁻¹ cm⁻¹) at the detection wavelength (λ>210 nm) than the parent[2.2.2]cryptand thus increasing its limit of detection in the finished[18F]radiopharmaceuticals before administered to patients for PET scan.

The following are the chemical structures of cryptate compounds of thegeneral formula (I), (II), (III), (IV), (V) for radiolabelingfluorinations, wherein cryptate is composed of UV detectable atwavelength greater than 210 nm and high molar absorptivity valuesgreater than 1000 M⁻¹ cm⁻¹ diaryl- and aryl-fused [2.2.2]cryptand andpotassium [¹⁸F]fluoride:

wherein R1, R2, R3, and R4 are each independently selected from H, or alower alkyl, or lower alkenyl, or alkoxyl, or benzyloxy, or ester, oramide, and or bromine.

SUMMARY OF THE INVENTION

In one embodiment of the present invention novel UV detectable (λ>210nm) potassium [¹⁸F]fluoride diaryl- and aryl-fused [2.2.2]cryptatecomplexes suitable for radiolabeling fluorinations.

A further embodiment of the method in the present invention is whereinthe di-substitutent in the di-substituted [2.2.2] cryptand is diaryl.Yet another embodiment of the invention is wherein the di-substituent inthe disubstituted [2.2.2] cryptand is dibenzo. Yet another embodiment ofthe invention is wherein the di-substituent in the disubstituted [2.2.2]cryptand is dinaphtho.

Yet, in a further embodiment of the present method the [¹⁸F]fluoride-complex is used to radiolabel a [¹⁸F] fluorinated specieswherein the radiolabdelled [¹⁸F] fluorinated species is used as animaging agent in a patient. Still another embodiment of the presentinvention discloses the imaging agent as being viewed within a patientby an imaging technique such as a positron emission tomography (“PET”)scanner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows absorbance spectra for acetonitrile reagent.

FIG. 2 shows absorbance spectra for methanol reagent.

FIG. 3 shows absorbance versus wavelength for K-22288, from 0.18mM-0.068 mM at pH=3

DETAILED DESCRIPTION OF THE INVENTION

The effect of substituents on macrocyclic molecules was first observedby Pedersen (Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 7017).Subsequently, many different moieties have been introduced into themacrocyclic backbone to modify the properties of the hosts, e.g., toincrease rigidity and lipophilicity, (Marchand, A. P.; Huang, Z.; Chen,Z.; Hariprakasha, H. K; Namboothiri, I. N. N.; Brodbelt, J. S.; Reyzer,M. L. J. Heterocyclic Chem. 2001, 38, 1361). The effect of increasedrigidity introduced by the incorporated moiety can be interpreted interms of preorganization. The principle of preorganization (Cram, D. J.in From Design to Discovery American Chemical Society, Washington D.C.,1991, p 9) states: “The more highly hosts and guests are organized forbinding and low solvation prior to complexation, the more stable will bethe complexes. The topology, along with ring size determines the degreeof preorganization of a specific structure for complexation. The generaltrend is that the two-dimensional structure develops into a threedimensional structure, wherein, for similar ring-size, the rigidity ofthe molecule increases. For example, rigidity increases along the series18-crown-6, [2.2.2]-cryptand. Increasing rigidity in this way restrictsthe ability of the ligand to undergo conformational reorganization. Thusmore rigid ligands are more highly “preorganized”. Since the host mustundergo conformational adjustment to provide a proper bindingenvironment during the host-guest interaction. Thus, preorganization ofa ligand, which is associated with its topology, rigidity and solvation,becomes important. For a specific guest, the more highly preorganizedligand requires less conformational change and thus pays minimal energycost for conformational adjustment.

Increasing rigidity of the host the more highly preorganized host andthe more highly host and guest are organized for binding the more stablethe complexes will be.

In order to attain high, yet selective binding of a potassium ionchelator some rigidity in the system such as the ionophore“dibenzo-[2.2.2] cryptand” was considered necessary.

The cavity size of [2.2.2] cryptand (2.8 A° in diameter) closely matchesthe size of potassium cation (2.66 A°).

Substituted [2.2.2] cryptands, such as dibenzo[2.2.2] cryptand (VII)possess a guest binding site (ionophore) having heteroatom Withnonbonding electron pairs such as nitrogen, capable of binding potassium(K⁺) selectively in its cavity. VII as phase transfer reagent (PTR) inthe synthesis of [18F]fluoride cryptate complexes for radiolabelingfluorinations will have improved detectability which will facilitatereliable assessment of PTR in the emerging direction of automated QCtesting platforms. VII has strong UV absorbance at λ>210 nm wavelength.Molar absorptivity values for VII are high across a wide range of pH,4100 M⁻¹cm⁻¹ at pH 2.4-3.0 (272 nm), and 4400 M⁻¹cm⁻¹ at pH 6.2-6.6 (276nm). K-222 has its absorbance maximum ˜200 nm where there is significantissues with solvent interference.

The synthesis of dibenzo-cryptand [2.2.2]; namely4,7,13,16,20,23-hexaoxa-1,10-diaza-19(1,2),24(1,2)-dibenzabicyclo[8.8.6]tetracosaphane(VII) is outlined in Scheme 1. The commercially available 2-nitrophenol(I) was chosen as a starting material. Treatment of two equivalents of(I) with 1,2-dibromoethane and potassium carbonate in dimethyl formamide(DMF) afforded 1,2-Bis (2-nitrophenoxy)ethane (II). Reduction of (II)with 10% Palladium-on-charcoal as the catalyst produced the aminoderivative (III). The diamine (III) was reacted with3,6-dioxaoctanedioyl dichloride (1,2-ethylene-O,O-diglycolic acidchloride) in tetrahydrofuran (THF) at high dilution conditions intetrahydrofuran (Dietrich, B.; Lehn, J. M.; Sauvage, J. P.; Blanzat, J.Cryptates. X. Syntheses and physical properties ofdiazapolyoxamacrobicyclic systems. Tetrahedron 1973, 29, 1629) to givethe lactam (IV). The lactam (IV) was reduced with Lithium AluminumHydride (LiAH4) in THF to give the azacrown (V) (Previously reported byde Silva, A. P.; Gunaratne, H. Q. N.; Samankumura, K. R. A. S. A newbenzo-annelated cryptand and a derivative with alkali cation-sensitivefluorescence. Tetrahedron Lett. 1990, 31, 5193-5196). Subsequenttreatment of (V) with 3,6-dioxaoctanedioyl dichloride gave (VI) whichupon reduction with diborane in tetrahydrofuran (Pettit, W. A.; Iwai,Y.; Berfknecht, C. F.; Swenson, D. C. Synthesis and structure ofN¹-e-benzo-4,7,13,16,21,26-hexaoxa-1,10-diazabicyclo[8.8.8]hexacos-23-yl-N²-phenylthiourea.Derivative of a bifunctional complexing agent. J. Heterocycl. Chem 1992,29, 877) furnished the cryptand (VII) (Naguib, Y M A. Molecules 2009,14, 3600-3609).

Di-substituted [2.2.2] cryptand possesses a guest binding site(ionophore) having heteroatom with nonbonding electron pairs such asnitrogen, capable of binding potassium (K⁺) selectively in its cavity.

Cryptand is a phase-transfer agent used to complex [¹⁸F] fluoride innon-aqueous environment to form [¹⁸F] fluoride cryptate complexessuitable for performing radio-labeling reactions to generate [¹⁸F]fluorinated species to be viewed through an imaging agent such asPositron Emmision Tomography (“PET”) and that a [¹⁸F] fluorinatedspecies defined herein comprises chemical or biological [¹⁸F]fluorinated compounds for use as imaging agents. Several approaches forincorporating ¹⁸F in biomolecules are described in the followingreferences: Kuhnast, B., et al. (2004) J. Am. Chem. Soc., 15, 617-627;Garg, P. K., et al. (1991) Bioconj. Chem., 2, 44-49; Lee, B. C., et al.(2004) J. Am. Chem. Soc., 15, 104-111; Chen, X., et al. (2004) J. Am.Chem. Soc., 15, 41-49; Glaser, M., et al. (2004) J. Am. Chem. Soc., 15,1447-1453; Toyokuni et al. Bioconjug. Chem. (2003) 14: 1253-9; andCouturier, O., et al. (2004) Eur. J. of Nuc. Med. and Mol. Imaging 31,1182-1206).

The present invention is not to be limited in scope by specific toembodiments described herein. Indeed, various modifications of theinventions in addition to those described herein will become apparent tothose skilled in the art from the foregoing description. Suchmodifications are intended to fall within the scope of the appendedclaims.

Various publications and patent applications are cited herein, thedisclosures of which are incorporated by reference in their entireties.

REFERENCES

Jewett et al, “Multiphase Extraction: Rapid Phase Transfer of [18F]Fluoride Ion for Nucleophilic Radiolabeling Reactions,” Appl. Radiat.Isot., vol. 39, No. 11, pp. 1109-1111, 1988

No-Carrier-Added (NCA) ARYL [¹⁸F] Fluorides Via the NucleophilicAromatic Substitution of Electron-Rich Aromatic Rings,” Ding et al.Journal of Fluorine Chemistry vol. 48, pp. 189-205 (1990)

The Synthesis of 6-[¹⁸F] Fluoro-L-Dopa by Chiral CatalyticPhase-Transfer Alkylation, ” C. Lemaire et al., J. Label Labelled Cpd.,Radiopharm 42 (1999) S113-S115

F-18 labeled biomolecules for PET studies in the neurosciences, Ding YS,Journal of Flourine Chemistry, 101:(2)291-295 Feb. 2000

Proton Irradiation of [180]02: Production of [¹⁸F]F2 and[¹⁸F]F2+[¹⁸F]OF2, Allyson Bishop et al., Nuclear Med. Biol. 1996, 23,189-199

4-[¹⁸F]Fluoroarylalkylethers via an improved synthesis of n.c.a.4-[¹⁸F]fluorophenol,“T. Ludwig et al., Nuclear Medicine and Biology 29(2002) 255-262

Babb, D.A., et al., “Synthesis ofHydroxymethyl-Functionalized-Diazacrowns and Cryptands,” Journal ofHeterocyclic Chemistry 23:609-613 (1986)

Blasius, E., et al., “Preparation and Application of Polymers withCyclic Polyether Anchor Groups,” Pure & App. Chem. 54(11):2115-2128(1982)

Bradshaw, J.S., et al., “Stable Silica Gel-Bound Crown Ethers. SelectiveSeparation of Metal Ions and a Potential for Separations of AmineEnantionmers,” Journal of Inclusion Phenomena and Molecular Recognitionin Chemistry 7:127-136 (1989)

Bradshaw, J.S., et al., “Silica gen-bound aza-crowns for the selectiveremoval and concentration of metal ions,” Pure & Appl. Chem.61:1619-1624 (1989)

Krakowiak, K.E., et al., “Syntheses of the Cryptands. A Short Review,”Israel Journal of Chemistry 32:3-13 (1992)

Krakowiak, K.E., et al., “One-step Methods to Prepare Cryptands andCrowns Containing Reactive Functional Groups,” Journal of HeterocyclicChemistry 27:1011-1014 (1990)

Krespan, C.G., “Funtionalized Macroheterobicyclic Compounds,” Journal ofOrganic Chemistry 45:1177-1180 (1980)

Montanari, F., et al., “Hydrocymethyl Derivatives of 18-Crown-6 and[2.2.2] Cryptand: Versatile Intermediates for the Synthesis ofLipophilic and Polymer-Bonded Macrocyclic Ligands,”, J. Org. Chem.,47:1298-1302 (1982)

Dietrich, B., “Cryptands,” in Comprehensive Supramolecular Chemistry,Atwood et al. eds., Jean-Marie Lehn--Chairman of the Editorial Board,New York: Pergamon, 1996, vol. 1, G.W. Gokel, ed., pp. 154-157, 186, 192

What is claimed is:
 1. A cryptate compound of the general formula (I),(II), (III), (IV), (V) for radiolabeling fluorinations, wherein cryptateis composed of UV detectable at wavelength 272 nm and molar absorptivityvalues selected from group of 42222, 40625, 41429, 41667, 41818, 42000,41250, 42857 M⁻¹cm⁻¹ diaryl- and aryl-fused [2.2.2]cryptand andpotassium [¹⁸F]fluoride:

wherein R1, R2, R3, and R4 are each independently selected from H, or alower alkyl, or lower alkenyl, or alkoxyl, or benzyloxy, or ester, oramide, and or bromine.
 2. The cryptate of claim 1 comprisesdibenzo[2.2.2]cryptand.
 3. The cryptate of claim 1 comprisesdinaphtho[2.2.2]cryptand.
 4. A method of using a cryptate of claim 1 toradiolabel [¹⁸F]fluorinated species.
 5. The method according to claim 4,wherein the radiolabeled [¹⁸F]fluorinated species is viewed by animaging technique.
 6. The method according to claim 5, wherein theimaging technique is a PET scanner.
 7. A method of synthesizingPotassium [¹⁸F]fluoride cryptate complexes of the general formula (I),(II), (III), (IV), (V) by combining [¹⁸F]fluoride anion with UVdetectable at wavelength 272 nm and molar absorptivity values selectedfrom group of 42222, 40625, 41429, 41667, 41818, 42000, 41250, 42857M⁻¹cm⁻¹ diaryl and aryl fused [2.2.2]cryptand and potassium carbonate:

wherein R1, R2, R3, and R4 are each independently selected from H, or alower alkyl, or lower alkenyl, or alkoxyl, or benzyloxy, or ester, oramide, and or bromine.
 8. The cryptate complex of claim 7, wherein thediaryl and aryl fused [2.2.2]cryptand comprises dibenzo[2.2.2]cryptand.9. The cryptate complex of claim 7, wherein the diaryl and aryl fused[2.2.2]cryptand comprises dinaphtho[2.2.2]cryptand.
 10. A methodcomprising cryptate complex according to claim 7 to radiolabel[¹⁸F]fluorinated species.
 11. The method according to claim 10, whereinthe radiolabeled [¹⁸F]fluorinated species is viewed by an imagingtechnique.
 12. The method according to claim 11, wherein the imagingtechnique is a PET scanner.